This invention relates to electrical wiring box arrangements. More particularly, it is concerned with electrically non-conductive wiring box arrangements having provision for the electrical grounding of a device mounted in such wiring boxes.
Generally, the electrical grounding of metallic wiring boxes or of wiring devices mounted in such boxes does not present a problem. It is a common practice to provide metal wiring boxes and certain wiring devices, such as duplex outlets, with a grounding lug for the direct attachment of the ground conductor wire of an electrical cable, although some wiring devices such as switches are not generally provided with such a grounding lug.
Duplex outlets of a type described in U.S. Pat. No. 3,032,736 to Howells which provide an electrical connection within the device between the grounding pin socket and the mounting yoke are now in common use. The firm mounting of such devices in a metallic wiring box results in electrical grounding of both the device and the box whether the ground conductor wire of the cable is attached to the ground lug of the box or the device, provided however, that there is firm electrical contact between the wiring box and the wiring device mounting yoke. Firm electrical contact may be a problem under some conditions because of the oversized mounting screw openings regularly provided in wiring device mounting yokes.
Numerous arrangements have been described in the prior art to insure firm electrical contact between the mounting yoke of a wiring device and a metal wiring box including clips, springs, or other devices which firmly engage the mounting screw. However, the increasing use in the building trades of plastic or other electrically non-conductive wiring boxes has created the need for some means of electrically grounding both the mounting yoke and mounting screw of wiring devices mounted in such boxes. The possibility arises in some situations that the wiring device mounting screw may become energized by coming into contact with a current-bearing conductor of the electrical cable. In such circumstances, the mounting screw or metal faceplate in contact with the mounting screw become potential sources of electrical shock hazard. This problem is especially acute when the wiring device is not provided with a grounding lug, as in the case of a switch device.
Clip devices of a type which rigidly attach to the wall of an electrically non-conductive wiring box and which provide an attachment lug for the ground conductor of the electrical cable are known in the art. Some of these clip devices provide for a tight fit between the mounting screw and the clip device and thus assure efficient electrical contact between the screw and the clip. However, because they are mounted rigidly to the wiring box wall, these devices generally make no provision for alignment of the mounting screw as it passes through the clip device into the screw-receiving member of the wiring box wall. When the mounting screw is driven through the clip device and into the box wall screw receiving member, improper alignment of the two results in considerable strain placed upon the assembly.
When the mounting screw of a wiring device is retained in the box wall by the simple expedient of a threaded hole in the soft wall of a plastic wiring box, this strain can result in stripping the threads, leading to unsatisfactory mounting of the wiring device in the box. Moreover, such improper alignment can result in poor electrical contact between the wiring device mounting screw and the yoke member of the wiring device resulting in poor electrical grounding.